1. Field of the Invention
This invention relates generally to high temperature polymers. In particular it relates to new polyimides formed from the reaction of aromatic dianhydrides containing isopropylidene connecting group between the aromatic rings with asymmetric amide diamines, whereby soluble, high strength, thermo-oxidatively stable, semicrystalline polyimides are obtained.
2. Description of the Related Art
Polyimides are condensation type polymers commonly synthesized by the reaction of aromatic diamines with aromatic dianhydrides. The intermediate polyamide-acid is either thermally or chemically dehydrated to form a polyimide which has a repeat unit of the general type ##STR1## where Ar is a suitable tetravalent aromatic radical which may be as simple as 1,2,4,5-tetrasubstituted benzene or a more complex structure having the general structure: ##STR2## where X is a bond or is a member selected from the group consisting of O, S, SO.sub.2, C.dbd.O, and C(CH.sub.3).sub.2, or Ar may be any other appropriate tetravalent radical; and where Ar' is a divalent aromatic organic radical.
Synthesis and characterization of polyimides has been extensively reported in the literature. The preparation of wholly aromatic polyimides by reaction of an aromatic dianhydride with an aromatic diamine, followed by thermal cyclization was first reported in 1963 [G. M. Bower and L. W. Frost, Journal of Polymer Science, Al, 3135(1963)]. Several reviews on polyimides have been published [C. E. Sroog, "Polyimides" in Encyclopedia of Polymer Science and Technology, (H. F. Mark, N. G. Gaylord, and N. M. Bikales, Ed.), Interscience Publishers, New York, 1969, Vol. 11, pp. 247-272; N. A. Adrova, M. I. Bessonov, L. A. Laius, and A. P. Rudakov, Polyimides, Technomic Publishing Co., Inc., Stamford, Conn., 1970; and D. Wilson, H. D. Stenzenberger, and P. M. Hergenrother, Ed., Polyimides, Blackie and Son, Ltd., London, 1990].
Polyimides heretofore known to the art are generally extremely insoluble after conversion from the polyamide-acid state to the polyimide form. Polyamide-imides with a carbonyl bridge in the dianhydride portion have been reported to be insoluble in organic solvents [J. F. Dezern, Journal of Polymer Science, A, 26, 2157(1988)]. As reported by F. W. Harris and L. H. Lanier in Structure-Solubility Relationships in Polymers, (F. W. Harris and R. B. Seymour, ed.), Academic Press, New York, pp. 183-198 (1977), a flexible linkage, such as a perfluoroisopropyl group in the dianhydride portion of the polymer improves the solubility characteristics of the polyimides. However, asymmetry in the connecting groups was not comprehended.
A primary object of the present invention is to provide soluble polyimides which can be dissolved in certain solvents and the solutions can be characterized and employed for further fabrication of the polyimides. In this way, polyimide films, coatings and the like can be produced without the necessity of using a polyamideacid intermediate with a follow-on conversion step. This is highly advantageous, because it permits the application of polyimide coatings to articles which might be damaged by heating or chemical conversion techniques.
Another object of the present invention is to provide high molecular weight polyamide-acid solutions that yield optically transparent, creasable, free-standing films and coatings in the fully cured form.
Another object of the present invention is to provide fully cured polyimide films with enhanced solubility in common organic solvents such as N,N-dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP).
Another object of the present invention is to provide high temperature stable aromatic polyimides which exhibit improved thermooxidative stability.
Another object is to provide polyimides that form well-consolidated moldings.
A further object is to provide polyimide films that exhibit high glass transition temperatures (&gt;300.degree. C.).